This invention relates to vertical pumps and particularly to nuclear reactor coolant vertical pumps with check valves.
In nuclear steam supply systems well known in the art, a reactor vessel contains fuel assemblies with nuclear fuel therein which produce heat in a commonly understood fashion. A coolant, which in fast breeder reactors may be liquid sodium, is circulated through the reactor vessel in heat transfer relationship with the fuel assemblies therein transferring heat from the fuel assemblies to the coolant. The coolant may then be conducted by a piping network to a heat exchanger and back to the reactor vessel tracing a path that is generally referred to as a primary loop while the coolant flowing through such a primary loop is referred to as a primary coolant or primary fluid. While passing through the heat exchanger, the primary coolant transfers heat to a secondary coolant or fluid. The secondary coolant may then be conducted to a steam generator that produces steam in a manner well known to those skilled in the art. The path traced by such a secondary coolant is generally referred to as a secondary loop. In many commonly known nuclear steam supply systems, there are three primary loops disposed symmetrically with respect to the reactor vessel. In order to circulate the primary coolant through these loops a coolant pump is provided in each such primary loop to pump the primary coolant through each primary loop.
During reactor operation, the three coolant pumps simultaneously pump primary coolant into the reactor vessel where the three primary coolant streams intermingle and pass in heat transfer relationship with the fuel assemblies therein. From this common pool of primary coolant, the primary coolant exits the reactor vessel under pressure into the remainder of the three primary loops. While the three primary loops function cooperatively under normal reactor conditions, under abnormal conditions the interconnection of the primary loops may result in damage to the system.
One such abnormal condition that may result in damage to the system is the failure of one of the coolant pumps while the other coolant pumps remain operating. In this situation, the operating coolant pumps may cause primary coolant to be conducted through the primary loop of the non-operating pump in reverse direction of its normal flow thereby causing the non-operating coolant pump rotor to turn in the opposite direction for which it was designed. This reverse rotation of the rotor of the coolant pump can cause severe damage to the coolant pump as is well known in the art. Although prevention of reverse flow through the non-operating coolant pump when the other pumps are operating is important, it is not the only consideration. Another important consideration is that in the event that all pumps fail simultaneously such as in a plant power failure, the primary loop paths must remain open to allow natural circulation of the primary coolant through the primary loops to facilitate cooling of the reactor vessel core.
One device known to prevent reverse flow when one coolant pump fails and to allow natural circulation when all coolant pumps fail is a type of swing valve that is placed in the piping network. This type of valve consists of a substantially circular metal flap attached by a hinge arrangement to the inside of a horizontal segment of piping such that under reverse flow the metal flap pivots about the hinge into an acute angle with respect to the hinge thus blocking the flow path. However, when all coolant pumps are not operating, the metal flap hangs from the hinge in a substantially vertical attitude without contacting the side of the pipe opposite the hinge thereby allowing natural circulation through the primary loop by allowing coolant to flow between the metal flap and the side of the pipe opposite the hinge because the natural circulatory flow is not sufficient to force the metal flap into the acute angle necessary to block flow. While this device does solve some of the reverse flow problems, it creates additional problems in that the hinge-metal flap attachment creates a wearing surface and a surface susceptible to self-welding under high temperature coolants which thereby demand frequent maintenance attendance.
In addition to being capable of solving the flow problems, when the primary coolant is liquid sodium the device must be capable of being completely drained for inspection and removal because any remnant of liquid sodium in the device that may become exposed to oxygen will burn violently when so exposed to oxygen. Furthermore, the device must be able to withstand the severe thermal transients present in a nuclear steam supply system without substantially increasing the length of the primary loop or substantially increasing the cost of the system. While there are types of check valves known in the art that are capable of preventing reverse flow, they are not capable of totally solving the flow problems in nuclear reactor steam supply systems.
There are many check valves in the art that allow flow in both directions under appropriate conditions. These check valves generally consist of a float member having a first end manufactured to conform to the shape of the valve seat and having a second end formed into a winged configuration capable of spanning a valve opening, opposite the valve seat, for allowing flow through the valve opening and between the winged configuration. Under normal conditions, a fluid is allowed to flow through the valve by passing through the winged configuration; however, under certain pressure conditions the first end of the float member is forced against the valve seat thereby preventing flow through the valve. While these valves do perform necessary functions, they may not be of an appropriate configuration for purposes that require a specific configuration for optimum operational efficiency.